where \(C_b\)  is the curve in \(J\)  corresponding to \(b \in B\)  and \(i_b \colon C_b \rightarrow J\)  is the inclusion. A computation using Riemann-Roch without denominators for regular embeddings (cf. [1]}) shows that the map in Equation REF  is a group homomorphism. An immediate corollary is the following.
